Why do I have an undefined reference error when linking for an object in the same archive?

Question

I have a C++ source mycpp.cpp and a C source myc.c. The C source contains a function myCFunc(), which is called from the C++:

extern "C"
{
#include "my_c.h"
}

void aCppFunction()
{
    myCFunc(stuff, and, things);
    ...
}

The two sources are compiled (to .o), and then archived (to .a), and then indexed, so no linking occurs:

ar qc thing.a *.o
ranlib thing.a

Later in the wider software build, this component archive is linked into a binary, and I get this error:

/usr/bin/ld: /path/to/mycpp.cpp:55: undefined reference to `myCFunc(unsigned char const*, unsigned int, unsigned int)'

I have extracted thing.a and can quite clearly see myc.c.o and mycpp.cpp.o, so I search them for the function:

> nm myc.c.o | grep myCFunc
00000000000001d0 T myCFunc
> nm -C mycpp.cpp.o | grep myCFunc
                 U myCFunc(unsigned char const*, unsigned int, unsigned int)

Can you see what is happening here? Why does the nm result for the C++ include the list of argument types, but the C result not? It seems to me that the function myCFunc() is defined in myc.c.o, which is part of thing.a along with mycpp.cpp.o, and that myCFunc's header is included correctly.

Please help!

Answer 1

The unlikely answer, based on j6t's invaluable comment:

Clearly, the caller of myCFunc does not see that name as extern "C", because the error message contains the parameter types. If it were seen as extern "C", the error message would only say undefined reference to myCFunc. – j6t

... is that what I did not show above, due to my C++ inexperience, is that the C header was included at the end of a train of other C++ headers, without using extern "C".